#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

#define RBUFSZ 4096
#define WBUFSZ 4096

static void die_perror(const char *what, const char *path, int exit_code) {
    int saved = errno;
    char msg[512];
    int n;
    if (path) {
        n = snprintf(msg, sizeof(msg), "%s: %s: %s\n", what, path, strerror(saved));
    } else {
        n = snprintf(msg, sizeof(msg), "%s: %s\n", what, strerror(saved));
    }
    if (n > 0) (void) write(STDERR_FILENO, msg, (size_t) n);
    _exit(exit_code);
}

static void xwrite_all(int fd, const char *buf, size_t len, const char *path) {
    size_t off = 0;
    while (off < len) {
        ssize_t n = write(fd, buf + off, len - off);
        if (n < 0) {
            if (errno == EINTR) continue;
            die_perror("write failed", path, -1);
        }
        off += (size_t) n;
    }
}

static long long parse_ll(const char *s) {
    char *end = NULL;
    errno = 0;
    long long v = strtoll(s, &end, 10);
    if (errno != 0 || end == s || *end != '\0' || v < 0) {
        errno = EINVAL;
        return -1;
    }
    return v;
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        const char *usage =
                "Usage: lab1_var_thirds_line <input.txt> <output.txt> <max_replacements>\n"
                "Replace every third non-newline byte in each line with a space; counter resets after LF.\n";
        (void) write(STDERR_FILENO, usage, strlen(usage));
        return -1;
    }

    const char *in_path = argv[1];
    const char *out_path = argv[2];
    long long cap = parse_ll(argv[3]);
    if (cap < 0) die_perror("invalid max_replacements", argv[3], -1);

    int in_fd = open(in_path, O_RDONLY);
    if (in_fd < 0) die_perror("open input failed", in_path, -1);

    mode_t perms = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;
    int out_fd = open(out_path, O_CREAT | O_WRONLY | O_TRUNC, perms);
    if (out_fd < 0) die_perror("open output failed", out_path, -1);

    char rbuf[RBUFSZ];
    char wbuf[WBUFSZ];
    size_t wlen = 0;

    long long total = 0;
    long long col = 0;
    int replacing_enabled = 1;

    for (;;) {
        ssize_t n = read(in_fd, rbuf, sizeof(rbuf));
        if (n > 0) {
            for (ssize_t i = 0; i < n; i++) {
                unsigned char c = (unsigned char) rbuf[i];

                if (c == '\n') {
                    if (wlen == sizeof(wbuf)) {
                        xwrite_all(out_fd, wbuf, wlen, out_path);
                        wlen = 0;
                    }
                    wbuf[wlen++] = '\n';
                    col = 0;
                    continue;
                }

                unsigned char outc = c;

                col++;

                if (replacing_enabled && (col % 3 == 0) && total < cap) {
                    outc = ' ';
                    total++;
                    if (total == cap) replacing_enabled = 0;
                }

                if (wlen == sizeof(wbuf)) {
                    xwrite_all(out_fd, wbuf, wlen, out_path);
                    wlen = 0;
                }
                wbuf[wlen++] = (char) outc;
            }
        } else if (n == 0) {
            if (wlen > 0) {
                xwrite_all(out_fd, wbuf, wlen, out_path);
                wlen = 0;
            }
            break;
        } else {
            if (errno == EINTR) continue;
            die_perror("read failed", in_path, -1);
        }
    }

    if (close(in_fd) < 0) die_perror("close input failed", in_path, -1);
    if (close(out_fd) < 0) die_perror("close output failed", out_path, -1);

    char res[64];
    int m = snprintf(res, sizeof(res), "%lld\n", total);
    if (m > 0) (void) write(STDOUT_FILENO, res, (size_t) m);

    return 0;
}